Lets say I have a wallet.dat backup, and I continue making transfers etc..
My pc crashes alot so the hdd could be damaged at some point.
If it would crash would i still have my coins since i saved the wallet.dat file earlier? Or do I have to make a backup every xamount of time
It depends on what wallet you are using.
Assuming you are using Bitcoin-Qt, you should create regular backups.
Bitcoin-Qt pre-generates 100 addresses that are included in your backup.
It then uses an address from that pool of 100 for change every time you send a transaction, and it uses an address from that pool of 100 every time you click the "New Address" button.
So, if you keep track of the number of times you click the "New Address" button, and you keep track of the number of transactions you send, then you'll need to create a new backup before the sum of those two numbers exceeds 100.
Since the wallet.dat file might be damaged when you back it up, it's a good idea to keep multiple recent backups. I generally suggest backing up every time the sum of transactions sent and addresses generated exceeds 25, and keeping copies of the 3 most recent backups.